51st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference<BR>18th
12 - 15 April 2010, Orlando, Florida
AIAA 2010-3063
VABS-IDE: VABS-Enabled Integrated Design Environment
(IDE) for Efficient High-Fidelity Composite
Rotor Blade and Wing Design
Patrick Hu1
Advanced Dynamics Corporation, KY 40511
Wenbin Yu2
Dept. of Mechanical and Aerospace Engineering, Utah State University, UT 84322
Dewey Hodges3
Daniel Guggenheim School of Aerospace Engineering, George Institute of Technology, GA 30332
Jieun Ku4
Independent Contractor, WA 98198
Abstract
This paper presents the development of a high-fidelity, yet efficient and easy-to-use,
composite rotor blade and wing section integrated design environment (IDE) to
facilitate rapid and confident aeromechanics assessment during conceptual design
stages. A well-known technical barrier for composite rotor blade and wing section
design is the lack of a user friendly, efficient and high-fidelity design tool to realistically
represent the blade section at the conceptual level. This limitation prevents designers
from accurately yet efficiently generating sectional properties, easily invoking
comprehensive analyses, and rapidly and confidently predicting the stress distribution.
As a result, aeromechanical analysis (e.g. for stability, loads, and vibration) is
unfortunately left out of the conceptual design phase. In order to overcome this
technical barrier and limitation, we propose to improve the functionalities of VABS
(Variational Asymptotic Beam Section analysis), the best proven technology for realistic
composite rotor blade analysis, and seamlessly integrate it with a versatile CAD
environment, a robust optimizer, and a general-purpose postprocessor, all of which are
specially tailored for blade and wing section design. The initial capability has been
established in the present study, and the full capability of a VABS-enabled IDE will
enable the efficient, high-fidelity composite rotor blade and wing section design in the
near future.
I.
Introduction
T
he design of rotor blade structures is a specialized skill that is labor and computationally intensive, requiring an
iterative process between cross-sectional analyses and rotorcraft comprehensive analyses. The former are
necessary for providing the sectional properties needed in the latter; the latter are needed for assessment of
stability, loads, and vibration. Present schemes in practical engineering for conceptual rotor blade design are
typically based on the use of historical data and simplified models for performance.
Aeromechanical analysis (e.g., for stability, loads, and vibration) is unfortunately left out of the conceptual
design phase because of the computational burden and the lack of detailed information about the system. For
1
President, Principle Scientist, [email protected], Senior Member AIAA.
Associate Professor, Senior Member of AIAA.
3
Professor, Fellow of AIAA.
4
Independent Contractor, Member of AIAA.
2
Copyright © 2010 by Advanced Dynamics Inc. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
example, detailed sectional properties along the blade are needed to determine the trim state and to assess stability
and loads. In turn, detailed sectional properties require intricate details about the cross-sectional geometry and
materials. As shown by Ku et al. [1], it is possible to make use of the blade properties themselves as design
variables, but this technique is too new to have found its way into industry practice. Moreover, the blade section
properties found to be ideal for the aeromechanical design may not be attainable in terms of current manufacturing
practice or available materials. This means that yet another iterative loop must be introduced. Because of the lack of
knowledge of design details at the early stages, serious aeromechanical problems are frequently not discovered until
the design space is significantly narrowed. At that time, it is very costly to change or remedy the design.
Currently, one of the main technical barriers in industry is that rotor blade designers lack an efficient, user
friendly, high-fidelity design tool to realistically represent the blade section at the conceptual level. This limitation
prevents designers from accurately yet efficiently generating the sectional properties, easily invoking comprehensive
analyses, and rapidly and confidently generating stress information within the material. The solution is to develop
high-fidelity design tools that are efficient and easy to use for cross-section design of composite rotor blades and
wings of future systems.
To create a high-fidelity, efficient and easy-to-use, composite rotor blade and wing section design environment
for rapid and confident aeromechanics assessment during conceptual design stages, a reliable and accurate crosssection analysis tool will be needed. Chapter 1 of the book by Hodges [2] reviews the history of such tools. There
are relatively few from which to select:
• ANBA – developed by Giavotto et al. (1983) [3]: This code was the original cross-sectional analysis,
developed mainly at the Polytechnic Institute of Milan, Italy. All the I/O is in Italian, although a version
with English I/O, called NABSA, was created in the early 1990s. Written in FORTRAN 77, it has neither
been updated nor maintained, and it lacks the benefit of feedback from a large user group. Finally, since it
is based on the Saint-Venant principle, it lacks a unifying basis for use in nonlinear analysis or for end
effects.
• Research code (name unknown) developed by Kosmatka [4]: This code, subject of Prof. John Kosmatka’s
PhD dissertation in 1989, has not been used extensively outside his group other than in a few applications,
e.g. [5]. Because it is also based on the Saint-Venant principle, it lacks a unifying basis for use in a
nonlinear analysis.
discussion of these differences.
• Research code (name unknown) developed
by Jung et al. [6]: This code is for thin- and
thick-walled beams. Unfortunately, a rotor
blade cross-section, which is a built-up
structure, is more complicated than a thin- or
thick-walled beam. Unlike its finite element
counterparts, this code cannot model both
simple and complex sections, and thus has
very limited value in anything other than a
preliminary design environment. Essentially
same approach is used in the beam analysis
on which Zhang and Smith base their design
methodology [7].
• VABS: Without question VABS has clear
advantages over all other approaches. It
should be noted that VABS, UM/VABS and
SectionBuilder all use the same underlying
theory. Differences among these codes are
Figure 1. VABS Technology.
in the I/O, the programming language, and
recently added features; see below for
During the last decade, significant advances have been made in the general-purpose modeling of helicopter rotor
blades. The state of the art is embodied in powerful comprehensive codes such as RCAS, CAMRAD II, and
DYMORE (the main rotorcraft comprehensive codes). To deal with realistic composite blades, however, these codes
must be supplied with realistic blade properties from a general cross-sectional analysis. The efficient, high-fidelity
cross-sectional analysis tool, VABS, originally developed at Georgia Tech, is the only tool capable of realistic
modeling of initially curved and twisted anisotropic beams with arbitrary sectional topology and materials. Relative
to 3D analyses, two to three orders of magnitude in computing time can be saved using VABS, with little loss of
accuracy. VABS has been under development for over 15 years and currently is used extensively in the rotorcraft
industry.
The advantages of VABS over other technologies have been clearly demonstrated by virtue of its generality,
accuracy, and efficiency. All US major helicopter companies and research labs have requested VABS. Boeing
Helicopter’s assessment is that “VABS has been available for several years as an integral component in the rotor
design process. VABS was selected over other tools based on its superior ability to handle the cross-sectional stress
analysis of rotor blades. There has been and continues to be significant testing and validation of VABS at the
company.”5 Boeing has incorporated VABS into its Common Structures Workstation to be used throughout the
entire company. VABS is designed to model structures for which one dimension is much larger than the other two
(i.e., a beam-like body), even if the structures are made of composite materials and have a complex internal
structure. VABS implements a rigorous dimensional reduction: from a 3D elasticity description to a 1D continuum
model (see Figure 1). VABS carries out a “cross-sectional analysis” by finite element discretization over the crosssection of the equations of the variational-asymptotic method [8]. All the details of the cross-sectional geometry and
material properties are included as inputs to calculate both structural and inertial coefficients. These properties can
be directly imported into comprehensive analyses to predict the aeromechanical behavior of the rotorcraft. Once the
comprehensive analysis has been invoked, pointwise 3D distributions of displacement, strain and stress over the
cross-section also can be calculated by VABS. Details of the theory behind VABS are found in chapter 4 of the book
by Hodges [2].
II.
Software Configuration of VABS-IDE
Briefly, this project has developed a VABS-IDE with the following software components:
1) Advanced PreVABS;
2) Advanced VABS technology to facilitate integration and automation;
3) Advanced robust optimization methodology;
4) A novel VABS-IDE with
• A versatile preprocessor (CAD) and;
• A general purpose postprocessor (Visualization) tool.
Figure 2 demonstrates the basic structure of this VABS-IDE. The following chapters will give the detailed
description of the methodology and approach in developing this VABS-IDE.
Figure 2. Basic Software Configuration of VABS-IDE.
A.
Advanced PreVABS Technology
The meshing of the blade and wing cross section is one of the critical functionalities of the IDE, and the meshing
functionality must be powerful, flexible, yet needs minimum user interference. Based upon the requirements for
realistic rotor blade and wing section conceptual design and the PreVABS matlab code developed by Prof. Yu’s
group at Utah State University, ADI team has developed the meshing algorithms and implemented it into an objectoriented C++ code, which is straightforward to integrate into the VABS-IDE developed by Advanced Dynamics.
5
Private communication from Kurt Kuhn, Matt Cawthorne, and Dick Blystone of Boeing Helicopters.
B.
Advanced VABS Technology
VABS was originally designed to run as a standalone code and its error handling, memory allocation/deallocation, and I/O were handled with this use in mind. However, to facility integration with other modules in the
proposed design environment, we need to modify VABS to be a callable library. In present study, we have
restructured VABS from a main program to subroutines which can be called by external programs. We called it
Advanced VABS to distinguish it from the old version. This modification mainly contains the following three
aspects:
1) Instead of terminating the code when critical errors arise (and output error messages are sent to the screen),
the code has been modified so that, when critical errors encountered, VABS returns control to the calling
program, along with corresponding error messages.
2) To avoid memory leak due to multiple runs, VABS has been modified so that all the dynamic arrays are
allocated at the time needed and de-allocated when we are done with its use whenever VABS is called.
3) All the inputs/outputs are localized at the I/O processing subroutines. VABS can be quickly invoked by
passing the necessary arguments to VABS.
In present study we have systematically made the aforementioned three changes to VABS to enable a plug-n-play
type capability for VABS, so that it can be seamlessly integrated with the proposed design environment. This highlevel modularization also allows us to keep the development of VABS independent from the development of the
design environment, and also will allow future enhancements to be quickly pasted into the design environment. We
have modified VABS to be two dynamic link libraries (DLLs).
C. Advanced VABS Technology for Facilitating Automation
To accurately represent a composite rotor blade section, VABS requires not only the common finite element
data set (nodal coordinates/elemental connectivity/material properties) but also information for the layup angles
and the orientation of the composite layer for each element. For highly curved layers, VABS requires nine numbers
for nodal orientations of the composite layer for each element. The biggest difficulty in preparing the inputs for
VABS is to automatically associate each material number with layer and provide the data set for the orientation of
the composite layer, and then have such information propagated to all the elements. Instead of modifying the
VABS directly, we believe this difficulty can be better resolved using a preprocessor specially. To this end, we
have developed a design driven pre-processing computer program, PreVABS as discussed in previous section, for
efficiently generating VABS inputs for realistic blades by directly using the design parameters such as CAD
geometric outputs and the spanwise and chordwise varying cross-sectional laminate lay-up schema. The
information available to rotor blade designers includes geometric data and cross-sectional laminate schema, and
some spanwise information including initial twist/curvature and tapering. These are the design variables for the
cross-sectional design. Based on this basic information, PreVABS can generate finite element mesh for all
functional components of the blade section including skins, webs, and other parts. Such information can be passed
to Constitutive.dll to carry out a constitutive modeling to get the sectional properties. Then after these properties
are used in a comprehensive rotorcraft analysis code (such as RCAS or DYMORE) to calculate the global
behavior, we are ready to use Recovery.dll to recover the pointwise distribution of stress and strains for each cross
section for detailed investigation.
D. Advanced Optimization Methodology
Optimization methodology has been used to explore feasible design options without overlooking potentially
superior designs that may not be recognized otherwise by eliminating man-in-loop iterations. Furthermore, utilizing
optimization methodology enables the capability to integrate multidisciplinary criteria in the early design stage by
producing a robust design solution. However, optimization is also known for its shortcomings: it can result in
excessive computation time due to analysis tools, and optimization of certain systems (e.g. rotor systems in
particular) is often ambiguous due to the nature of their design space. The optimization methodology discussed
here is intended to demonstrate a solution that overcomes the downsides of blade design optimization by using a
computationally efficient tool (VABS) along with a hybrid methodology that incorporates both gradient and nongradient methods.
E. VABS-enabled IDE GUI Environment
The graphical user interface is the look-and-feel of the IDE software and is developed based on Tcl/Tk and VTK
to provide graphical elements for the user to interactively set up a VABS based analysis. The required input data is
created using a PreVABS module, which in turn uses more primitive geometric and lamina schema data. The GUI
enables the standard file I/O, creation of 2-D geometry models, manipulation of view, and invocation of individual
software modules.
The workspace of the IDE is divided into four major areas, as shown in Figure 3 below. The top border holds
menu items, icons for initiating file I/O, graphical view manipulation, geometry model building and also a row of
icons for selecting the particular module to use. A portion of the area on the right is used to provide buttons,
choices, text fields and text areas for using functions available in an individual module. Every module can have one
or more interfaces which are accessed in a tree-like manner, making the use highly intuitive and error-free. The
center area, which occupies most of the real-estate of the IDE is the graphical view area showing the model
geometry, discretized model showing elements and data visualization. At the bottom there is a fourth area, called
the response/feedback area which shows any diagnostics and feedback from the IDE system to the user. This
design provides a clutter-free smoothly transitioning set of interfaces through which the user can move from model
acquisition to post-processing.
Figure 3. The Workplace of the IDE.
F. Integration of Versatile Preprocessor / CAD environment
The basic feature of the versatile preprocessor/CAD environment has been implemented in this VABS-IDE by
using the custom made OpenGL library from Advanced Dynamics. The full capability of versatile
preprocessor/CAD environment includes the following features:
•
Read a CAD data for geometry from a file such as an .IGES file
•
Create or modify realistic sections using basic geometry entities such as key points, lines, and
surfaces.
•
Create or modify realistic sections using generic wing layouts such as airfoil shape, skin with a
certain thickness, web, D-spar, etc.
•
Mesh realistic sections using quadrilateral ( and a little triangular for transition) elements
•
Integrate with VABS and execute VABS with a click on the button in the environment
•
Create a 3D finite element model of a realistic blade
•
Graphical view manipulation like zooming, panning, rotation and reset are available, both from
keyboard and mouse buttons
G. Integration of PreVABS
The PreVABS module uses the lamina schema data read through the GUI or, directly, and creates a suitable
grid at the push of a button. PreVABS as supplied, was a standalone console software application which could read
files in a batch mode, and write the grid files and VABS input files. It was appropriately modified to run inside the
IDE. A DLL was created which could be loaded on demand.
The user starts by activating the [PreVABS] button from the module choice icons. Processing is started from its
interface by clicking on the PreVABS button. Depending on the fineness of the grid and the desired number of
elements the processing time with the PreVABS can vary. A model involving close to 100,000 elements can get
computed on a modern PC in a couple of minutes. Once PreVABS finishes building the element topology, it shows
the number of nodes and elements in the feedback area. After this, the user can perform grid visualization.
H. Grid Visualization Tools
Visualization functions are implemented on top of a custom made OpenGL library from Advanced Dynamics.
Graphical view manipulation like zooming, panning, rotation and reset are available, both from keyboard and
mouse buttons. These functionalities are essential for visual examination of elements, especially in areas where
skew grid elements are likely to be generated.
I. Integration of VABS Analysis
VABS analysis uses the input files created by PreVABS. The objective was to provide push-button execution of
the VABS analysis, and the same has been realized by writing bridge software code to load Constitutive Modeling
and Recovery libraries, and running appropriate routines.
In the current implementation VABS runs as a “black-box” process, with any diagnostics displayed in the
feedback area of the IDE. With the fully developed IDE one could interactively change the parameters used in the
VABS solution process.
VABS analysis is also inherently a CPU intensive process, and could take several minutes depending on the
size of the model.
J. Textual Display of Stiffness Matrix and Stress/Strain Recovery
VABS produces stiffness matrix data from its constitutive modeling and stress/strain data through its recovery
calculations. These can be displayed in the VABS interfaces upon a successful completion of the analysis.
K. Integration of an Optimization Tool
As of this writing only GUI provisions are made for invoking an optimization tool. Depending on the software
available for this step further integration work will be done.
L. Visualization of Data Fields
Visualization of field variables, i.e. quantities defined at each of the nodes or element centers, is provided from
a set of interfaces. Individual variables, e.g. ply angle, can be selected from a choice element, and various kinds of
plots be made through appropriate selections. Contour plots, both line and filled contours are available. Contour
levels are displayed in the graphics window, and can be turned off.
Other than the provision for eye-balling the results, the IDE also offers tools to save plots in many different
formats for sharing and presentation.
III. Demonstration of the Initial Capability of VABS-IDE
Points, lines, and surfaces are the basic geometric elements. For any CAD environment with geometry
modeling ability, creating and manipulating these basic geometric elements are the foundation for further creating
and manipulating complex geometric objects. In order to simplify the data input and efficiently use current
resources, the integrated IDE also has the ability to read in the standard CAD file, such as IGES file, to get the
geometry. These functionalities have been implemented in the IDE to demonstrate the capability of the underdeveloped IDE. Further integration of the geometry modeling with the PreVABS still needs more work and is not
implemented yet.
[Undo] and [Redo] buttons are used during geometric modeling for undoing, or re-doing an action, e.g. creation
or deletion. Figures 4 and 5 show the panels of creating curves by specifying three points, or by importing series of
points from data files.
Figure 4. Curves Creation Window (Three Points, Accept Spline).
Figure 5. Curves Creation Window (Importing Points from Data Files).
Geometry models containing points, lines and curves can be imported into the IDE using an IGES file. Editing
geometry models read from an IGES file is implemented through integration of the imported model with natively
created elements.
IGES files can be read by selecting [Read iges files] from the top-level menu, as shown in Figure 6. The
imported geometry is also shown in Figure 7. The finite element mesh can be generated automatically by using
preVABS module as shown in Figure 8. The re-covered displacement distribution is shown in Figure 9 for a
model rotor blade.
Figure 6. Importing IGES File with Curves and Surfaces.
Figure 7. PreVABS Created Mesh.
Figure 8. PreVABS Interface after Element Creation and Zooming.
Figure 9. Recovered 3-D Displacement.
IV. Conclusion and Recommendation for Future Work
Several conclusions can be drawn and recommendation for future work is provided below:
(1) Now, VABS can be used both as a standalone application and a callable library. It can be connected with any
other languages which can call dynamic link libraries (DLLs).
(2) To facilitate the integration and automation, we transplanted PreVABS, the originally Matlab preVABS code
into the VABS-IDE using C++, a much more efficient language than Matlab, thus avoiding the use of Matlab
license when the fully developed IDE is commercially available on the market.
(3) We have also developed a MSC/Patran like integrated CAD environment for VABS-IDE. MSC/Patran is the
leading pre- and post-processing commercial environment for CAE simulation. Most of the end users are
familiar with MSC/Patran. Thus, it will be fast for them to understand and control the usage of the developed
integrated CAD environment. The proposed initial capabilities have been implemented, with straightforward
menus and mouse clicks.
(4) The two-stage local optimization conducted on several other problems was coupled with global optimization.
The integrated method needs to be developed and whole procedure needs to be integrated into IDE future
work.
(5) The initial capability of this VABS-IDE can be used to visualize (1) the geometry modeling; (2) the meshing
of the unstructured triangular and quadrilateral elements; and (3) data distribution through line and surface
plots such as design objectives versus design variables, or stress distribution along a certain direction.
In summary, an initial, innovative and promising VABS-IDE has been developed with a user-friendly and
convenient GUI. This VABS-IDE has the fundamental and independent functions for blade designing engineers to
easily create the geometry of modeling blades, to easily do the meshing by Advanced PreVABS, to easily perform
VABS analysis, to easily perform the two-level optimization, and to easily show the schematic results including
meshing, stress and strain recovery. Obviously, this VABS-IDE will become a simple, useful, and powerful tool in
practical rotor blade and wing section design.
Acknowledgement
This work is supported by Army SBIR Phase I Contract No. W911W6-09-C-0016 and Army SBIR Phase II
Contract No. W911W6-10-C-0026, Mr. Gerardo Nunez is the technical monitor. Special thanks should go to Boeing
Company, Bell Helicopter, Sikorsky Aircraft and Aerovironment Inc for their valuable comments and strong
supports on this project.
References
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11
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